Service cables that deliver power to residential buildings from a power supply typically include a neutral conductor which provides a return path for the electric current to the power supply; the power supply is usually a utility transformer within an electric distribution system. When a portion of the power frequency (60 Hz) electric current returns to the utility transformer via paths other than the neutral conductor, a net current is created in the service cable and the alternate current path in the building. Net current is defined as the instantaneous sum of all of the currents that are flowing in an electrical conductor (or group of electrical conductors) forming the service cable. As described in EPRI Report EL-6509 "Pilot Study of Residential Power Frequency Magnetic Fields", the net current is one of the primary causes of power frequency magnetic fields in residential buildings. Net currents in residences typically range from zero amperes to as much as ten amperes. Results from the EPRI Report TR-102759-VI "Survey of Residential Magnetic Field Sources", show that the median home in its study had magnetic fields that exceeded 1.2 mG in homes with metallic water lines and 0.5 mG in homes with plastic water lines in the center of the room with the highest field. This is based on the level that is exceeded 5% of the time during the 24-hour measurement period.
Net current on the cables supplying current has been a long-standing situation. It is only recently that the general public has become aware of possible health effects from exposure to electric and magnetic fields (EMF). Although substantial questions remain about the possible health effects of power frequency magnetic fields, the public is interested in eliminating or reducing their exposure to the fields. It may be desirable, therefore, to eliminate, or at least to reduce, these fields inside of and in the vicinity of the buildings to which the electrical power is supplied. Since net currents mostly are the sources of these magnetic fields, it would be desirable to provide a means for controlling these net currents.
In the communication and data transmission arts, as distinguished from power transmission, it is well known to surround an electric cable with a ferrite or other magnetic substance of different cylindrical or rectangular shapes to reduce high frequency noise on the electrical cable. Noise suppression devices (or noise absorbers) are disclosed, for instance, in U.S. Pat. Nos. 5,003,278, 5,200,730 and 5,864,814. These devices are formed from a ferromagnetic material and are attached around the electric cable to suppress electric noise that is generated within an electronic device or that enters the electronic device from outside through the electric cable. The cables, which use this type of product on data transmission electronic circuits, usually process frequencies from computer sources. This is an altogether different purpose from that of the present invention.
Attempts were made in the art of power transmission for reducing currents induced in the sheath or other metallic coverings of cables carrying alternating or pulsating currents. This sheath current, in addition to the losses in transmission it causes, has an appreciable effect on the heating of the cable; hence reduces the permissible current in the conductor. In order that these sheath currents may be reduced and at the same time to permit the grounding of the cable, U.S. Pat. No. 1,752,320 discloses cores of magnetizable material provided with properly designed coils placed in proximity to the cable, so as to be magnetized by currents flowing in the conductors of the cable. The coil, or winding, of the core is capable of producing an electromotive force and is connected in such a manner as to reduce or, if desired, to neutralize the effect of the electromotive force induced in any specified length of sheath.
In order to obtain a balance between the sheath electromotive force and the electromotive force induced in the winding or coil, the number of turns in the coil are varied, or an air gap in the magnetic circuit is provided, or the dimensions of the core itself are varied, or the position of the core and coil with respect to the cable is altered.
However, none of these prior art technologies concerned reducing a power magnetic field in the vicinity and within residential buildings by controlling net current flowing in conductors of electrical systems of residential buildings and by keeping the sum of instantaneous current is flowing in all conductors of the cable equal to zero.
There have been recent attempts to minimize net currents in the service entrance cables caused by currents returning to the power system via the grounding connection to the metallic water pipe in residential homes. These attempts have been undertaken by some people to reduce net currents in the electric service cable and plumbing because of concerns that increased magnetic fields in their home might possibly be detrimental to their health. The most common approach to reducing net currents has been to install an electrically insulating pipe coupling in the water pipe outside of the house, as for example, at the shut off valve at the edge of the property. Insulating couplings for this application are commercially available. The primary disadvantage of this approach is that it increases the electrical impedance between grounding connection in the house back to the electrical supply system and may result in an increased shock hazard.